Previous work has yielded a multi-function scintigraphic imaging system capable of planar, SPECT and, most recently, PET imaging. During this reporting period, the PET capability of the system was further evaluated on the bench top and in preliminary small animal studies. The principle limitation of the present ultra high resolution, single slice NaI(Tl) PET scanner was found to be detection efficiency. Although this limitation in some cases could be overcome by administering high tracer doses, low detection efficiency precludes certain important types of imaging studies where large doses cannot be given, and will, in general, cause the acquired images to be noise limited. As a consequence, an elaborate Monte Carlo simulation of the system was written to allow evaluation of alternative scintillation materials and detector geometries that might increase detection efficiency substantially. Based on these calculations, a strategy was developed that should raise detection efficiency by a large factor while retaining most of the advantageous performance characteristics of the present system.